Engine Coolant Temperature Sensor

The Engine Coolant Temperature (ECT) Sensor is a Negative Temperature Coefficient (NTC) Thermistor that monitors the engine coolant temperature. The ECT Sensor operates within a 5 volt DC reference range, and provides a linear input signal to the Powertrain Control Module (PCM) that is based upon the measured engine coolant temperature. The PCM uses data from the ECT Sensor to calculate air-fuel mixture, ignition timing, and fuel injector pulse width. In some instances, the ECT signal is used to control the engine cooling fans and the operation of the air conditioning (A/C) system.

Removal & Installation

Specific to:

Lexus ES250 1990-1991

Lexus ES300 1992-1999

Disconnect the negative battery cable.

Disconnect the ECT sensor connector.

Using a 19 mm deep socket wrench, remove the ECT sensor and gasket.

To install:

Installation is the reverse of removal. Tighten the sensor to 14 ft. lbs. (20Nm).

Testing

Specific to:

Lexus ES250 1990-1991

Lexus ES300 1992-1999

NOTE

Refer to the Electrical Wiring Diagram for component and connector locations, connector views, and circuit-specific information.

Connection & Wiring Diagnosis

Specific to:

Lexus ES250 1990-1991

Lexus ES300 1992-1999

Refer to the Electrical Wiring Diagram for component and connector locations, connector views, and circuit-specific information.

Many intermittent open or short circuits may be caused by wiring harness and connector movement due to vibration, engine torque, bumps and rough pavement, etc.

Test the wiring harness and connectors by performing the following tests:

Move the related connectors and wiring while monitoring the appropriate scan tool data.

Move the related connectors and wiring with the component commanded ON and OFF. Using a suitable the scan tool, observe the component operation.

With the engine running, move the related connectors and wiring while monitoring component operation.

If harness or connector movement affects the data displayed, the component and system operation, or the engine operation, inspect and repair the harness or connections as necessary.

Test the connector terminal pins and/or wiring by performing the following tests:

Inspect for incorrect mating of the connector halves, or terminals not fully seated in the connector body.

Inspect for improperly formed or damaged terminals and test for incorrect terminal tension.

Inspect for poor terminal to wire connections including terminals crimped over insulation. This requires removing the terminal from the connector body.

Inspect for corrosion or water intrusion. Pierced or damaged insulation can allow moisture to enter the wiring. The conductor can corrode inside the insulation with little visible evidence. Look for swollen and/or brittle sections of wire in the suspect circuits.

Inspect for wires that are broken inside the insulation by gently pulling on suspect sections of wiring.

ECT Circuit Testing

Specific to:

Lexus ES250 1990-1991

Lexus ES300 1992-1999

Use a suitable scan tool, a Graphing Multi-Meter (GMM), or a Digital Volt-Ohm Meter (DVOM) in order to view the ECT data.

ECT Sensor logic is relative to the scale of resistance: when the coolant temperature is low, the voltage is increased. When the coolant temperature is high, the voltage is decreased.

Use a suitable pyrometer or thermometer, as well as visual observation to verify that the cooling system is operating properly. If a cooling system fault is evident, repair as required before continuing.

If a DVOM is being used, additional information may be acquired by taking measurements at the sensor connector as well as the PCM connector. If a significant voltage drop is measured (greater than 0.5 volts DC), check the wiring harness and connections for corrosion, poor pin connections, or damaged wires.

If all engine wiring and pin connections are confirmed, disconnect the ECT Sensor to verify the signal with a DVOM, and verify PCM communication before replacing the ECT Sensor.

Fig. ECT Sensor Range Chart

ECT Sensor Strategy

Specific to:

Lexus ES250 1990-1991

Lexus ES300 1992-1999

The ECT Sensor provides a linear input signal to the Powertrain Control Module (PCM) for a number of engine control system calculations. The ECT signal is compared with Manifold Absolute Pressure (MAP), Mass Air Flow (MAF), Intake Air Temperature (IAT), and Fuel Trim (HO2S) in order to determine the correct air-fuel mixture for measured operating conditions. In some cases the ECT Sensor input is used in concert with the A/C Pressure (ACP) Sensor input in order to determine cooling fan and A/C system operation.